1. Field of the Invention
The present invention relates to an exposure method and apparatus used for transferring a mask pattern onto a substrate in lithography processes to produce, for example, semiconductor devices, image-pickup devices (e.g., CCDs), liquid-crystal display devices, or thin-film magnetic heads. More particularly, the present invention relates to step-and-scan or other scanning exposure methods and apparatuses, in which a mask and a substrate are scanned synchronously relative to a projection optical system, thereby sequentially transferring a mask pattern onto the substrate.
2. Description of the Related Art
In production of semiconductor devices and so forth, one-shot exposure type projection exposure apparatuses, i.e., steppers, have heretofore been used, and step-and-scan type projection exposure apparatuses are also used these days, in which a reticle as a mask and a resist-coated wafer are synchronously moved relative to a projection optical system, thereby enabling exposure to be carried out for a shot area wider than the effective field of the projection optical system. In general, projection exposure apparatuses use a projection optical system which has a large numerical aperture (N.A.) and a shallow depth of focus. Therefore, it is necessary in order to transfer fine circuit patterns at high resolution to effect focusing such that the surface of the wafer coincides with the image plane of the projection optical system by an autofocus system.
Scanning exposure type projection exposure apparatuses such as the step-and-scan type similarly need to bring the surface of the wafer into focus at the image plane. However, in the scanning exposure type, the surface of the wafer is continuously scanned relative to a projection area (hereinafter referred to as "illumination field") where a part of the reticle pattern image is projected. Therefore, according to a known method, the focus position of the wafer surface (i.e., the position in the optical axis direction of the projection optical system) is detected only at predetermined detecting points in the illumination field. There has also been proposed a method in which the focus position is preread at detecting points preceding the illumination field, and the wafer surface in the illumination field is brought into focus to the image plane on the basis of information concerning the preread focus position and the focus position detected in the illumination field.
FIG. 19 illustrates exposure which is carried out to a wafer by a conventional step-and-scan type projection exposure apparatus. In FIG. 19, at the time of starting exposure, a slit-shaped illumination field 112, which is a projection area where a part of the reticle pattern image is projected, is at an approach start position preceding a shot area 111 on the wafer which is to be exposed. At this stage, the illumination field 112 is not illuminated by exposure light. Thereafter, the illumination field 112 starts to approach along a path shown by the arrow 113, and the shot area 111 is relatively scanned by the illumination field 112, thereby transferring the reticle pattern image onto the shot area 111. In actual practice, the wafer is moved through a wafer stage relative to the fixed illumination field 112. In FIG. 19, however, a system is illustrated such that the illumination field 112 moves relative to the wafer for the sake of explanation. Exposure light is applied after the illumination field 112 has reached the shot area 111.
In a system in which prereading is carried out during the scanning exposure, the focus position is continuously measured at detecting points 114A to 114E in the illumination field 112 by using a predetermined autofocus sensor. In addition, the focus position is continuously measured at detecting points 115A to 115C in a preread area preceding the illumination field 112 in the scanning direction. On the basis of the results of measurement of the focus position, an autofocus operation is executed by a servo system to effect focusing such that the surface of the wafer in the illumination field 112 coincides with the image plane, in the form of an operation including predictive control which is continuously executed from the approach start position.
When the shot area 111 is near the edge of the wafer, for example, focus position detecting points may be located outside the wafer when the illumination field 112 is at the approach start position. There are also cases where focus position detecting points lie in an area near the edge of the wafer where unevenness (undulation) is remarkable. In such cases, the focus position detecting points are moved to a flat area on the wafer, for example, by driving the wafer stage, and the wafer surface position is locked on the basis of the focus position measured at the flat area. In this state, approach of the illumination field 112 is executed.
In the conventional scanning exposure type projection exposure apparatus, at the time of starting exposure, the illumination field is not in a shot area which is to be exposed but lies at an approach start position preceding the shot area. Accordingly, in the system in which autofocusing is effected by measuring the focus position only at the detecting points in the illumination field, follow-up control for the focusing operation cannot satisfactorily be effected when there is a step in the shot area, for example. Therefore, a resolution failure or other problem may occur.
In the system in which an autofocusing operation is executed by effecting predictive control using prereading, if there is a steep change in unevenness on the surface in a shot area to be exposed, follow-up control may be impossible to effect on account of the mechanical response speed of the servo system, for example. In particular, when exposure is carried out for a partly-cut shot area near the edge of the wafer by scanning the illumination field relative to the wafer from the outside thereof, it is difficult to effect accurate follow-up control because of the steep change in unevenness on the wafer surface near the edge, and it is likely that exposure may be undesirably executed in a defocused state.
To avoid these problems, it may be considered to measure a distribution of focus positions in a shot area to be exposed in advance of scanning exposure. However, the detectable range of conventional autofocus sensors is limited within the illumination field or within an area with a predetermined width in the scanning direction beyond the illumination field. Therefore, to measure a distribution of focus positions in the shot area, the position of the wafer stage must be moved from the approach start point to a position where it is possible to measure the focus position in the shot area. Accordingly, it is necessary to repeat a series of operations, i.e., premeasurement of the focus position, start of approach, and scanning exposure, by moving the wafer stage for each shot area, and thus the throughput (productivity) of the exposure process is unfavorably reduced.
FIGS. 20(a) and 20(b) illustrate exposure conventionally carried out onto a wafer by using a step-and-scan type exposure apparatus similar to that used in FIG. 19, showing a case where an abnormal step is present in an approach section. Problems arising when there is an abnormal step in an approach section will be described below in detail with reference to these figures.
Referring to FIG. 20(a), when scanning exposure is started, an illumination field 121 is gradually accelerated (in actuality, the wafer is accelerated in the reverse direction) in an approach section (acceleration section) 122 leading to a shot area 120 as in the case of the process described in connection with FIG. 19. At this time, an autofocusing operation is continuously executed by a servo system to bring the surface of a wafer in the illumination field 121 into focus to the image plane on the basis of results of focus position detection carried out at detecting points in the illumination field 121 and at detecting points preceding the illumination field 121 in the scanning direction. Application of exposure light is started after synchronization of the wafer and an associated reticle has been completed in the approach section 122 and the illumination field 121 has reached the shot area 120, and the illumination field 121 is scanned relative to the wafer in an exposure section 123 along a path 126 shown by the arrow, thereby sequentially transferring a reticle pattern image onto the shot area 120. Thereafter, the application of exposure light is suspended, and the illumination field 121 is gradually decelerated in a deceleration section 124. In this case, if the scanning direction is reversed on the reticle side every time the exposure process shifts to the subsequent shot area, a useless movement is eliminated, and the throughput (productivity) can be improved. Therefore, it is common practice to reverse the scanning direction also on the wafer side in accordance with the reversion of the scanning direction on the reticle side every time the exposure process shifts to the subsequent shot area.
Thus, in the conventional step-and-scan type projection exposure apparatus, the focus position detection and the wafer surface position control based on the result of the detection are continuously carried out from the approach section 122 so that the wafer surface in the illumination field 121 can be accurately brought into focus to the image plane in the exposure section 123. However, if there is an abnormal step area 125W in the approach section 122 which is markedly different in the focus position from the surrounding area, as shown in FIG. 20(a). As a result of a foreign matter 125 present at the bottom of the wafer W as shown in FIG. 20(b), when the illumination field 121 passes over the abnormal step area 125W, the height of the wafer stage lowers a considerable amount in order to bring the wafer surface at this portion into focus at the image plane. Accordingly, when the illumination field 121 enters the shot area 120 after passing over the abnormal step area 125W, follow-up control for the wafer surface position cannot satisfactorily be effected. Consequently, exposure is started in a state where there is defocus between the wafer surface and the image plane. Therefore, a resolution failure or other problem may occur.